The present invention is directed toward furnace control and, more particularly, toward controlling the operation of a furnace and the combustion conditions within the furnace based upon electrostatic precipitator operating conditions.
Controlling furnace operation for optimum combustion and performance is of great importance in the industry. Ever since furnaces were first operated, optimizing combustion and performance have been desired goals, and various methods have been developed over the years in an attempt to achieve these goals. For example, as late as the middle of the twentieth century, shipboard operators utilized a method known as efficiency haze in an attempt to optimize boiler combustion. The efficiency haze method required a shipboard, or boiler, operator to visibly monitor the cloud of smoke emanating from the stack of the furnace. For boilers fired by oil, the boiler operator would reduce the excess air into the boiler until there was a faint but visible plume of smoke emanating from the stack. This faint but visible plume of smoke indicated to the boiler operator that not too much excess air was being used, stack heat losses were at a minimum, and most of the carbon present in the oil was being burnt.
Similarly, boilers fired by coal were also controlled by monitoring the visible plume of smoke emanating from the stack. However, coal-fired boiler operators would watch for a darkening of the visible plume of smoke. The darkening of the visible plume of smoke was caused by flyash from the coal and indicated to the boiler operator that unburned coal was being emitted, i.e., that combustion was not complete. The boiler operator would then modify the air flow to the boiler in order to optimize combustion of the coal. Other analysis techniques, such as the Orsat analysis and various other types of instrument analyses, were also developed to determine gas compositions and improve the operation of the furnace. However, these prior art analyses techniques were mostly performed in the stack, and adjustments were made only to the total airflow into the boiler.
Today, large utility boilers often have multiple oxygen sensors in the flue gas stream, typically located at the outlet of the economizer. The sensor outputs are made continuously available in the furnace control room, and airflow to the boiler may be modified based on the sensor outputs. Alarms may be provided for low oxygen even at only one sensor. The sensors provide signals representative of the oxygen concentration in various parts of the furnace, thus permitting adjustments to be made to increase or decrease the excess air on one or more sides, or one or more corners, of the furnace. However, oxygen concentration in the flue gas, which is sensed by the sensors, is not the total answer to the problem of optimizing furnace operation. For example, the oxygen concentration sensed by the sensors may be caused by an infusion of air into the boiler through bad seals and/or casing holes. Since this infused air usually occurs after combustion is quenched by temperature loss, it is not relevant to combustion conditions and, as such, may result in adjustments being made which actually degrade furnace operation. In other cases, coal feed pipes and coal particle size may become unbalanced within certain burners or regions of the furnace such that incomplete combustion problems do not appear as oxygen deficiencies, but rather as unburnt carbon problems.
Combustion monitors have also been developed in an effort to improve furnace combustion control. Combustion monitors typically take the form of a carbon monoxide (CO) monitor. Carbon monoxide is the most common gaseous by-product of incomplete combustion. The presence of CO often indicates that there is insufficient air for combustion, but CO may also be present in the flue gases due to other reasons, such as, but not limited to, poor air/fuel mixing, poor particle size grinding (assuming coal is used as the fuel), delayed ignition, and rapid cooling. While multiple CO monitors could be installed in various sections of the furnace to monitor and control combustion, such an array of CO monitors is expensive to install and operate. Further, gaseous CO monitors cannot determine if combustion of the solid or liquid components of the fuel is complete.
Other methods of determining the completeness of fuel combustion have been developed, again in an effort to optimize furnace operation. When coal is used as the fuel to be burned, one such method that has been developed is to measure the combustibles in the flyash. One prior method of measuring combustibles in the flyash is to observe the opacity and color of the smoke emanating from the stack (efficiency haze). However, observing smoke opacity and color sometimes only indicates how well the particulate collection equipment (electrostatic precipitator) is working, and at best only indicates total furnace operation rather than indicating how well various sections of the furnace are performing. It has also been found that substituting an automatic and continuous opacity monitor for an operator""s visual observations does not change matters much. While some work has been done on developing continuous and instantaneous flyash carbon instruments, they have not been very successful since standard methods of flyash collection and carbon or loss on ignition analyses are too slow for proper furnace control. Other methods that have been developed include optical absorption or emission characterization of the flyash particle clouds in-situ, but these generally still have to be correlated back to an implied carbon content of the flyash entering the electrostatic precipitator.
The ultimate NOx emissions will increase if the individual burner air/fuel ratio is increased. Corrosion of some of the boiler tubes may increase if the air/fuel ratio of particular burners is reduced too far. While operators usually attempt to maintain the air/fuel ratio the same for all burners by controlling all coal feeders at he same rate and all secondary air registers at the same location or by following the feeder speeds with secondary air adjustments to individual burners this procedure often does not result in uniform air/fuel ratios between the various burners. Even measurements of the secondary air flow and the primary air flow will, at times, result in errors in each as large as plus-or-minus 8%, and when these errors are coupled with coal feeder errors and different coal loading in the primary air to the different burners supplied by the same pulverizers, the air/fuel ratios for different burners can differ by 25% or more.
It is desirable to control the air/fuel ratios the same for all burners to allow minimum excess air operation without excessive tube corrosion, CO emissions, or carbon in the flyash. Minimum excess air operation is necessary to increase unit efficiency, reduce NOx emissions, and often simply to reduce the total air and flue gas flows which allows for higher capacity when a unit is air fan limited.
The present invention is directed toward overcoming one or more of the above-mentioned problems.
A method is provided for controlling the operation of a furnace. A furnace generally includes a boiler having a combustion zone, a plurality of burners burning a mixture of fuel and air in the combustion zone producing a gaseous by-product, and an electrostatic precipitator in fluid communication with the boiler removing particulates from the gaseous by-product. The method includes the steps of monitoring operating conditions of the electrostatic precipitator on a section-by-section basis, and controlling a select one or ones of the plurality of burners based upon the section-by-section monitored operating conditions.
The various operating conditions of the electrostatic precipitator which may be monitored on a section-by-section basis include, but are not limited to, spark rate, voltage recovery rate, power usage, rapping frequency, and the opacity of the gaseous by-product upon exit from the electrostatic precipitator.
In one form, each of the plurality of burners includes a primary air and fuel line and a secondary air duct. The primary air and fuel line supplies the mixture of fuel and air to be burned in a pre-select fuel/air ratio at a pre-select primary flow rate. The secondary air duct supplies a secondary flow of air to assist in the burning process. In this form, the controlling step includes modifying at least one of the pre-select fuel/air ratio, the pre-select primary flow rate and the secondary flow of air for a select one or ones of the plurality of burners based upon the section-by-section monitored operating conditions of the electrostatic precipitator.
The monitored sections of the electrostatic precipitator may be associated with the plurality of burners on a one-to-one basis, or may be associated with plural burners burning their respective fuel/air mixtures in particular regions of the boiler combustion zone.
In a preferred form, the fuel to be burned includes coal of various types, with the particulates in the gaseous by-product including flyash.
In another form, the fuel burned includes a fuel which produces solid particles during combustion and the solid particles are discharged in the gaseous combustion products as flyash.
In another form, the furnace is equipped with cyclones rather than burners. Such cyclones have primary air of an unusual type and secondary air, but the coal is not pulverized and the primary air does not carry the coal into the cyclone. Either the fuel flow or the secondary air flow to individual cyclones can be changed to alter the air/fuel ratio of the individual cyclones. There may be as few as one-tenth as many cyclones in a boiler as there are burners in a pulverized coal fired furnace of the same capacity.
In another form, the select one or ones of the plurality of burners are controlled to optimize furnace operation, reduce NOx emissions, reduce CO emissions, and/or reduce particulates present in the gaseous by-product.
In yet another form, the monitored sections of the electrostatic precipitator are assigned to the plurality of burners based on select criteria, with control of the plurality of burners based upon the monitored operating conditions of its assigned section of the electrostatic precipitator. Each section of the electrostatic precipitator may be assigned to a different one of the plurality of burners, or may be assigned to plural burners burning their respective fuel/air mixtures in particular regions of the boiler combustion zone. In this form, the adaptive process control software creates a dynamic model, which is then used to decide if there are statistically significant interactions between section-by-section monitored ESP conditions and specific burners.
In still another form, a dynamic process model is developed from the section-by-section monitored operating conditions of the electrostatic precipitator, and control of the plurality of burners is based upon variations in the dynamic process model.
Other aspects, objects and advantages of the present invention can be obtained from a study of the application, the drawings, and the appended claims.